1. Field of the Invention
The present invention relates to telescopic crane booms for mobile cranes, and more particularly, a hollow telescopic crane boom whose cross section includes an inverted U-shaped section and a convexo-concave section.
2. Description of the Related Art
Mobile cranes have telescopic booms which can handle heavy loads and can be extended to great heights. Although the load handling capability of a telescopic boom can be increased by simply increasing its size or bulk, such design solution is often undesirable as it entails a corresponding increase in the size and weight of the drive motor and mechanism of the mobile crane. Furthermore, state highway regulations require permits for operating vehicles, such as mobile cranes, whose weight exceeds a specified limit.
Prior art telescopic boom designs achieve good strength to weight ratios by employing hollow boom sections with rectangular, trapezoidal or triangular cross sections. Typically, these cross sections are made from structural steel plates having thickness of at least about 3/16 of an inch so that the boom would not easily buckle and become plastically deformed. To further increase the strength of these booms, stiffeners are added to the side walls of the booms so as to increase the shear carrying capacity of the side walls. These known boom sections are inexpensive to produce and are adequate for smaller cranes.
Another prior art telescopic boom is known from European Publication EP 0 499 208 B1. This European publication discloses a hollow boom cross section formed by an inverted U-shaped longitudinal section and a semi-circular or semi-elliptical longitudinal section. The inverted U-shaped section and the semi-circular section are welded to each other along the seams formed by longitudinal edges of these sections. The telescopic boom is mounted on the mobile crane such that when the boom is in a horizontal position, the inverted U-shaped section is disposed atop the semi-circular section. So configured, the inverted U-shaped section of the boom would be subjected to tensile stresses while the semi-circular section would be subjected to compressive stresses during use. This telescoping boom has high buckling strength due to the special geometrical configuration of the semi-circular or semi-elliptical section. It is, however, expensive to produce due to its semi-circular or semi-elliptical configuration.
According to European Publication EP 0 583 552 B1, this disadvantage can be overcome by forming instead a trough-like section having a plurality of strips affixed at an oblique angle to each other. This trough-like section however, has lower buckling strength than that of the semi-circular section due to the placement of strips located at the bottom-most portion of the trough-like section. In addition, the contour of the bearing member for supporting the telescoping boom sections must be adjusted for the progression or the sliding action of the trough-like section.
Accordingly, there is a need for a telescopic boom which is simple and inexpensive to produce and which is geometrically configured to have high buckling strength suitable for use in large cranes. There is also a need for a telescopic boom with easily adjustable bearing members for supporting the boom sections.